The additional magnification values are taken from the "known (printed on the side!!!)" magnification of the additional components in the microscope; for example, in the case of the older Nikon Optiphot 2 upright microscope the epi-fluorescence module has a fixed 1.25x mag lens in it. This lens compensates for the change in the tube length of the microscope (160mm non-infinity optics). The DIC attachment for this microscope had a similar compensating optic which also fitted between the objective revolver and the trinocular head, which may have been 1.25x, hence Jim Pawley mentions it.

Additionally, some "up-to-date" microscopes have a magnification lens that can be switched "in" at the users discretion. E.g. I think the Olympus IX-70/71 has a 1.5x and 2x magnifier that can be switched in and out. Other microscopes may also have this type of switchable component.

Typically, none of these components are recommend to be used when imaging in confocal mode, but are unavoidable in the case of the older microscope which have the optics built in.

This should be confirmed experimentally, like in Check Biorad Magnification. If no extra magnification is seen, uncheck both checkmarks on the form below. Each checkmark introduces a extra 1.25× magnification. Alternatively you can forget the checkmarks and introduce the total internal magnification (not including the objective's) in the correspondent entry field, replacing the default 53.2.